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Frog deformities are being detected at levels drastically above any
rate which might plausibly be thought to be natural.
[new
research on frogs]
Chapter 9 of Our Stolen Future discussed evidence that frog
populations in many parts of the world are declining and explored
the unproven but plausible hypothesis that endocrine disrupters
might be involved. The "frog story" has grown more dramatic since
OSF went to press, with alarming deformities now showing
up in frogs all across Minnesota and Wisconsin and in the St. Lawrence
River Valley in Quebec. Researchers in Minnesota have found grotesquely
deformed frogs at more than 100 sites. One animal had four legs
sprouting from its stomach; another had a leg growing out of its
neck. The most bizarre, perhaps, was a frog with an eye inside its
mouth. The animals appear to be victims of severely derailed development.
While scientists are exploring a number of theories to explain these
developmental deformities, including disruption caused by parasites
or by bacterial or viral disease, pesticides and synthetic chemicals
have become leading suspects.
William
Souder describes one deformity outbreak in A
Plague of Frogs:
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"The
scene at the Bocks' lake deteriorated in front of Hoppe's
(a herpetologist at the University of Minnesota) eyes. By
midsummer the lake had become a watery nightmare. Each successive
collection revealed a fresh perversion of nature. The frogs
reproducing along the Bocks' shoreline--where the family had
swum and caught their Sunday supper for years, and where Hoppe
had seen nothing out of the ordinary only weeks before--were
being ravaged. There were significant levels of deformity
in every species. The most severe abnormalities--the worst
yet seen anywhere in Minnesota--occurred among the mink frogs.
The
mink frogs at the Bocks' lake exhibited every type of deformity
previously seen at other places in the state--plus a ghastly
parade of new ones. Some were missing legs or leg parts, but
most had some sort of twisted addition to their normal complement
of limbs. Many had the strange skin webbings--Hoppe started
calling them "cutaneous fusions--that spanned the trailing
edges of the hind legs, preventing their full extension and
in the most extreme cases totally immobilizing the limbs.
Hoppe found it hard to look at these animals and not wonder
if they weren't likely to die by drowning.
Other
frogs had stumps of legs or partly formed limbs protruding
at odd angles from their hindquarters. There were legs that
split, branching off in two or more directions, as well as
legs that had extra feet, and feet that had extra toes. There
were tapering, leglike appendages sticking of from the rear
flank like wings on an airplane. One frog had a total of nine
legs."
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Some
scientific progress has been made on the central question of what
is causing these deformities, but it has been slow, halting and
contentious.
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Parasites can cause some of the
deformities seen and may be involved in the natural background
rate of deformities. This has been established through an elegant
combination of laboratory and field
research. Even the scientists who performed this research,
however, do not believe that parasites are the only cause.
- Field
and lab experiments with wood frogs
reveals that pesticides reduce the frogs' ability to resist parasitic
infection. Frogs in contaminated areas have higher rates of infestation
than those without contamination.
- Careful
studies of the geography of deformities, especially in SE Canada,
show a striking relationship with the distribution of agricultural
chemicals.
- Deformed
frogs have been found with no sign of parasitic infection.
- Water
from areas where deformities occur in greatest abundance has been
shown in experiments to produce deformities without any parasites
involved.
- Several
agricultural chemicals when applied to developing frogs in the
laboratory cause a range of deformities, including forms similar
to virtually all types seen in the wild.
Thus
the picture that is emerging is that frog deformities occur in some
areas as a result of parasitic infections. In other regions, indeed
where deformities are most common, parasites do not appear to
be a major contributor. In these areas, particularly the upper portions
of the US mid-West eastward to Ontario, agricultural pesticides
are clearly involved. Which chemical(s) and via what mechanism is
far from resolved.
Press
coverage...
Two
websites that monitor ongoing research:
Amphibian
deformities
Minnesota
Pollution Control Agency
For
an excellent overview of the search for the causes of frog deformities,
read William Souder's A Plague of Frogs.
A useful
essay
by Dr. Michael Lannoo on the scientific pursuit of frog deformities
can has been published on a website HMS Beagle.
New
research on deformed frogs:
Experimental
laboratory work by James La Clair et al. (1998) shows that
the breakdown products of a common insecticide causes deformities
in toads similar to those found naturally in frogs. More...
Johnson
et al. (1999) demonstrate that parasites can cause deformities
in experimental ponds and these correspond with patterns of deformities
observed in central coastal California. More...
A
study by Sower et al. published in October 2000 finds that
deformed frogs have hormones out of balance compared to normal frogs.
More...
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